1. Field of the Invention
The invention relates to an image forming apparatus using an electrophotographic process or an electrostatic recording process, and particularly to an image forming apparatus such as a copying machine, a printer or a FAX.
2. Description of the Related Art
In some of conventional image forming apparatuses such as copying machines, printers and facsimile apparatuses, for example, a toner image is formed on an image bearing member by the use of an electrophotographic process, and the toner image is transferred to a recording material.
FIG. 6 of the accompanying drawings schematically shows the construction of an image forming apparatus using such a conventional electrophotographic process, and in FIG. 6, the reference numeral 1 designates a photosensitive drum, the reference numeral 2 denotes a contact charger for charging the surface of the photosensitive drum 1, the reference numeral 3 designates a laser beam scanner using, for example, a semiconductor laser which is an exposing device as information writing means for forming an electrostatic latent image on the surface of the photosensitive drum 1, and the reference numeral 4 denotes a developing device.
When an image is to be formed in such a conventional image forming apparatus, the surface of the photosensitive drum 1 is first charged by the contact charger 2, whereafter a laser beam modulated according to an image signal sent from a host apparatus such as an image reading apparatus (not shown) to the image forming apparatus side is outputted by the laser beam scanner 3, and the uniformly charged surface of the photosensitive drum 1 is subjected to laser scanning exposure L (image exposure) at an exposing position b.
When such laser scanning exposure L is effected, the potential of that portion of the surface of the photosensitive drum 1 to which the laser beam has been applied drops, whereby an electrostatic latent image corresponding to the scanning-exposed image information is sequentially formed on the surface of the photosensitive drum 1.
Next, the electrostatic latent image is developed by the developing device 4, whereby a toner image is obtained on the photosensitive drum 1. Thereafter, the toner image is electrostatically transferred to the surface of a recording material (transfer material) P fed from a feed mechanism portion (not shown) at predetermined control timing at a transferring portion “d” constituted by a transfer roller 5d′ which is a transfer charger urged against the photosensitive drum 1 with a predetermined pressure force and the photosensitive drum 1.
That is, the recording material P fed to the transferring portion d is transported by being nipped between the rotating photosensitive drum 1 and transfer roller 5d′ and in the meantime, a transfer bias of the positive polarity which is opposite to the negative polarity which is the regular charging polarity of the toner is applied from a power source S3 to the transfer roller 5d′, whereby the toner image on the surface of the photosensitive drum 1 is sequentially electrostatically transferred to the upper surface of the recording material P.
Next, when it thus passes the transferring portion “d”, the recording material P to which the toner image has been transferred is sequentially stripped off from the surface of the photosensitive drum 1 and is transported to a fixing device, e.g. a thermal roller fixing device 6, where it is subjected to the fixing process of the toner image, and thereafter is outputted as an image-formed article (a print or a copy).
Now, the developing device 4 for visualizing the electrostatic latent image on the photosensitive drum 1, as shown in FIG. 6, is provided with a rotatable nonmagnetic developing sleeve 4b, a stationary magnet roller 4c inserted in the developing sleeve 4b, a developer coating blade 4d, a developer agitating member (screw) 4f for agitating and circulating a two-component developer having toner and carrier contained in a developing container 4a to thereby rub the toner and the carrier against each other and triboelectrify the developer, and a toner hopper 4g containing a supply toner (not shown) therein.
The developing sleeve 4b is disposed in proximity to and in opposed relationship with the photosensitive drum 1 with its closest distance to the photosensitive drum 1 kept constant. Also, this developing sleeve 4b is adapted to be rotatively driven in a direction opposite to the direction of rotation of the photosensitive drum 1 in a developing portion “c”, which is a portion opposed to the photosensitive drum 1.
Further, the two-component developer in the developing container 4a is attracted to and held on the outer peripheral surface of the developing sleeve 4b as a magnetic brush layer by the magnetic force of the stationary magnet roller 4c in the developing sleeve.
The two-component developer thus attracted to and held on the developing sleeve 4b is transported with the rotation of the developing sleeve 4b, and is adjusted into a predetermined thin layer by the developer coating blade 4d, whereafter in the developing portion “c”, it contacts with the photosensitive drum 1 and moderately rubs against the surface of the photosensitive drum.
Also, at this time, a predetermined developing bias, e.g. a vibration voltage comprising a DC voltage (Vdc) and an AC voltage (Vac) superimposed one upon the other, is applied from a power source S4 to the developing sleeve 4b. Thereby, the electrostatic latent image on the surface of the photosensitive drum 1 is visualized by the toner.
On the other hand, FIG. 7 of the accompanying drawings shows another construction of the conventional developing device 4, and as shown in FIG. 7, a regulation member 4d is in proximity to the developing sleeve 4b, and when the two-component developer passes this proximate portion, the charging of the toner “t” is adapted to be effected by the triboelectrification between the developing sleeve 4b and the regulation member 4d. 
Now, there has also been produced an image forming apparatus in which the process speed (corresponding to the rotating speed of the photosensitive drum, the developing sleeve 4b or the screw 4f) is changed in conformity with the kind of the recording material to thereby form an image. That is, when the recording material is plain paper, an ordinary speed mode in which the process speed is selected to an ordinary speed is adopted, and when the recording material is thick paper, a low speed mode in which the process speed is selected to a low speed is adopted, and when the recording material is an OHP sheet (light transmissive resin), a lowest speed mode in which the process speed is selected to the lowest speed is adopted.
When in such an apparatus, there is adopted a construction in which by the use of a magnet sensor 14 for detecting a change in the permeability of the developer in the developing device to thereby detect the density of the toner in the developer, the control of the supply of the toner to the developing device 4 is effected, the following inconvenience has occurred.
With the change of the image forming mode, i.e., the image forming speed, the transport speed (circulation speed) of the developer by the screws 4f, 4e is changed and at the same time, the flow speed of the developer flowing through an area opposed to the detecting surface of the magnetic sensor 14 is also changed, and in conformity with this change in the flow speed of the developer, the output from the magnetic sensor 14 changes greatly.
That is, when the image forming speed is changed, the output of the magnetic sensor 14 immediately before and immediately after the change of the image forming speed changes greatly in spite of the toner in the developing device 4 being not consumed and therefore, the control of the toner supply thereafter such as for the oversupply of the toner or the deficient supply of the toner could not be executed well.